From the early days of theorizing about multi-stage rockets, it has been realized that the best performance could be achieved by taking the concept to its limit—a continuous process of discarding empty portions of the fuel tanks. Several patents have been issued in this field, both for solid fuel, monopropellant and bi-propellant liquid fuel, but none has been applied to a production launcher. The conventional practice is to build a launcher from several stages, positioned one on top of the other, and to drop them off one by one once the propellant is exhausted in each one.
Prior art proposed various methods of reducing the weight of a rocket fuel tank during launch as a continuous process, as opposed to the above mentioned method of dropping off complete stages once their fuel is exhausted.
The method of achieving a continuous discarding of empty tank portions involves two basic issues:                1. Moving the bottom of the tank, carrying the combustion chamber and nozzle (hereafter called “piston”), up inside the outer tank wall to the top of the tank, as the fuel is consumed.        2. Discarding the redundant tank wall of the tank that extends aft of said moving piston.        
Regarding the first requirement of propelling the piston up, prior art has proposed either internal or external means, extending the whole length of the tank, for pulling the piston up to the top of the tank.
U.S. Pat. No. 4,703,694 proposes as one option a rack and pinion mechanism, with the rack extending internally the whole length of the tank. Another option proposes an internal cable for pulling the piston up.
U.S. Pat. No. 3,043,221 proposes tension members—very long springs or rubber cords—extending the whole length of the tank internally.
U.S. Pat. No. 3,067,575 proposes an internal cable pulling the piston up.
U.S. Pat. No. 3,127,739 proposes either an internal screw jack, an internal ratcheted rod, or an external system of racks and pinion. All these elements extend the whole length of the tank.
U.S. Pat. No. 7,210,282 proposes several external screw jacks, extending the whole length of the tank
Such piston moving elements have various drawbacks. Since the fuel tank is long, as it is in typical launchers, the weight of a rack, a ratcheted rod or a screw jack, probably made of steel, will be high. If it is internal to the tank, it will have to be compatible with the fuel, and good sealing will be required top and bottom, where it passes through the structure. An external system will be even heavier, since at least two pulling elements will be required. For example, a steel screw jack with a two inch diameter, running the length of a twenty meter tank, weighs over two hundred Kg.
There is also the problem of cutting said rod/rack/screw jack as it extends below the piston, in order to get rid of its weight.
U.S. Pat. No. 3,250,216 proposes a combustion chamber and nozzle for a solid rocket motor, where the combustion chamber is pushed up along the fuel tank by the thrust alone, with no control over its speed.
German Pat. No. DE102010011553 B4 also proposes that the thrust of the rocket motor is the only motive force for pushing the pistons up, without any control system.
Such a system may be good for a solid rocket motor, but for liquid/gel propellant, control is required over the pressure in the tank and therefore some motion control mechanism is needed.
Regarding the second issue, getting rid of the empty portions of the fuel tank, extending below the piston, all prior art proposed to dispose of the structure by burning it, either inside the rocket engine itself, or by the hot plume of the engine(s).
U.S. Pat. No. 4,703,694 proposes to burn the empty skin by the plume of the rocket motors, but in order to use the redundant skin as fuel for added thrust, a ram-rocket element is added, which also adds to the weight and complexity of the launcher.
U.S. Pat. No. 3,043,221 & U.S. Pat. No. 3,067,575 also proposes to burn the empty skin by the plume of the rocket motors.
U.S. Pat. No. 3,127,739 proposes a different method, according to which the structure of the tanks is fed directly into the special combustion chamber, and burns there together with a special solid fuel, carried specifically for this purpose. In another embodiment of said patent a more conventional combustion chamber is shown, but it still has to burn the tank structure, which is continually inserted into it.
U.S. Pat. No. 7,210,282 proposes pulling the rocket case into the combustion chamber for burning. An elaborate sealing system is required to prevent hot gas escape around the entrance to the combustion chamber.
German Pat. No. DE102010011553 B4 proposes that the redundant skin protruding below the rocket motor will be burned off, presumably by the rocket plume, but no details are given.
U.S. Pat. No. 3,308,624 is perhaps the most complex in this regard, as it proposes to mechanically grind the case to chips before burning them to provide thrust. A great amount of energy is required for this operation, which in turn requires special turbines to provide the power.
Another proposal (U.S. Pat. No. 4,723,736) involves installing several sub-stages of oxidizer tanks one above the other, forming the fuel tank by their inner surfaces. A piston carrying the rocket motor moves up inside the fuel tank as the fuel is consumed. The sub-stages are discarded once the oxidizer inside them is consumed, and the piston has moved to the higher sub-stage. A complex construction is required, and a system of tubes, valves and separation devices. The sealing between the sections, which is critical to create an internal fuel tank, is difficult to achieve.
There are several disadvantages to all those patented methods:                1. The structure must be made from a material that will burn easily, yet be safe for use, and will not burn ahead of the desired location. In the case of a solid fuel motor, the rate of case burning must match the rate of fuel burning, in order to keep the combustion chamber at a constant volume.        2. Some have suggested a composite structure in order to facilitate burning, but this may produce toxic gasses, which may not be acceptable.        3. Burning of the case inside the rocket engine, such as proposed by U.S. Pat. No. 3,127,739, means that a special rocket engine must be developed, instead of using an existing engine. An engine that must burn metal or composite materials may be difficult to develop, and in any case this will require investment of money and time in order to ensure steady and reliable burning.        4. Burning the case by the exhaust plume may have an effect on the propulsion of the launcher. An attempt to increase thrust by using the case as a fuel, such as proposed by U.S. Pat. No. 4,703,694, also requires a development of a special engine system.        5. The proposal involving sub-stages of oxidizer tanks is very complicated, requiring a system of tubes and valves, many tanks with their attachment and ejection elements, and more skin area than in conventional tanks, increasing weight.        